Transcript PowerPoint Example 2

Population genetic implications of post-glacial migration
in Carex cryptolepis (Cyperaceae)
Nathan Derieg and Leo P. Bruederle
Department of Biology, University of Colorado at Denver and Health Sciences Center, Denver, CO 80217
Introduction
Discussion
Pleistocene glaciations greatly influenced the modern
flora and fauna of Eastern North America. With the onset
of the Wisconsin Glaciation, climate change induced shifts
in species distributions, and continental ice sheets
overran those populations that where not extirpated by
changing environmental conditions (Pielou 1991). The
effects varied; some species persisted relatively close to
the ice front, while others seem to have survived in
southern refugia.
As expected, populations of C. cryptolepis exhibit
relatively low levels of genetic diversity and are highly
differentiated. Inbreeding can result in a loss of alleles,
but similar levels of inbreeding in C. cryptolepis and C.
lutea indicates an additional process acted to reduce
genetic diversity in C. cryptolepis. The decrease in
number of polymorphic loci and allelic variation at
polymorphic loci may have resulted from founder effects in
post-glacially established populations. The high degree of
population differentiation in C. cryptolepis relative to C.
lutea may reflect limited gene flow among populations
established after the last glacial maximum; however,
incomplete sampling of broadly distributed species can
inflate observed levels.
Retreat of the continental ice sheets allowed dispersal into
glaciated regions from the refugial populations. Range
expansion of this sort is expected to result in low genetic
diversity and a large degree of population differentiation
(Ibrahim et al. 1996).
Carex cryptolepis Mack. is a broad endemic distributed
across glaciated areas of northeastern North America,
occuring on sandy or organic substrates with neutral or
acidic pH and low calcium content (Fig. 1) (Crins and Ball
1989). Carex cryptolepis is a self compatible perennial
with caespitose, or clump forming, growth habit (Fig. 2). A
small number of populations are found south of the last
glacial maximum (LGM), e.g., the Edge of Appalachia
Preserve in southern Ohio.
Hypotheses
Populations of Carex cryptolepis occupying
formerly glaciated regions will exhibit
reduced genetic diversity.
While none of the populations included in this study were
likely refugial, further sampling along the southern
distributional margin, e.g., southern Ohio, may identify
such populations. Populations from higher latitudes will
help characterize the extent and pattern of genetic
diversity reduction in C. cryptolepis. Additionally, further
sampling will clarify the intraspecific phylogeography of C.
cryptolepis within the context of post-glacial migration.
Populations of C. cryptolepis will also be
highly differentiated.
Results
Allozyme analysis was performed to assess the impact of
post-glacial migration on the levels and apportionment of
genetic diversity within and among populations of C.
cryptolepis.
Methods
Soluble enzymatic proteins were extracted from 346
individuals representing ten populations of C. cryptolepis
(Table 1) (Bruederle and Fairbrothers 1986). Carex lutea,
putative sister taxon to C. cryptolepis, was utilized for
comparisons. Samples were stored at -70oC in the
UCDHSC Plant Systematics Lab. Allozyme
electrophoresis was conducted using 11% starch gels and
three gel-electrode buffer systems; thirteen substrate
specific stains resolved 18 putative loci (Bruederle and
Fairbrothers 1986; Bruederle and Jensen 1991; Kuchel
and Bruederle 2000). Observed allozyme phenotypes for
each individual were interpreted as genotypes following
Bruederle and Fairbrothers (1986), with loci and alleles
named following standard nomenclature.
GDA 1.1 (Lewis and Zaykin 2002) was used to generate
descriptive statistics, including: proportion of polymorphic
loci (P), mean number of alleles per locus (A), mean
number of alleles per polymorphic locus (Ap), observed
heterozygosity (Ho), expected heterozygosity (He), Nei’s
unbiased genetic identity, and Wright’s F-statistics. The
CONTML function of PHYLIP version 3.65 (Felsenstein
2005) was used to construct a phylogenetic hypothesis for
C. cryptolepis.
Allozyme data for ten populations indicates C. cryptolepis maintains low
levels of genetic diversity relative to the closely related C. lutea, a narrow
endemic from unglaciated North Carolina (e.g., P = 3.9% versus P =
21.1%) (Table 2). Expected heterozygosity and observed heterozygosity
were both lower in C. cryptolepis (He = 0.007, Ho = 0.004) than C. lutea (He
= 0.051, Ho = 0.029). Statistically significant deviations from HardyWeinberg Equilibrium were correlated with large positive fixation indices.
Mean inbreeding within populations (f = 0.49) was similar to that observed
in C. lutea (f = 0.44). Populations of C. cryptolepis were more
differentiated (FST = 0.86 versus FST = 0.40). CONTML analysis supports
recognition of C. cryptolepis as a distinct species and suggests the
presence of intraspecific lineages (Fig. 3).
P
5.56%
0.00%
0.00%
5.56%
5.56%
5.56%
16.67%
0.00%
0.00%
0.00%
3.89%
Ap
2.00
***
***
2.00
2.00
2.00
2.00
***
***
***
2.00
He
0.012
0.000
0.000
0.021
0.028
0.002
0.010
0.000
0.000
0.000
0.007
Ho
0.002
0.000
0.000
0.009
0.014
0.002
0.010
0.000
0.000
0.000
0.004
f
0.814
0.000
0.000
0.566
0.516
0.000
-0.021
0.000
0.000
0.000
0.491
0.855
C. lutea
21.11%
2.15
0.051
0.029
0.440
0.404
clump-forming
sedges
14.15%
2.06
0.043
C. cryptolepis
AL
AF
CA
ML
PC
SR
SV
TM
TP
WC
mean
0.412
Table 2. Summary of genetic diversity statistics, including: proportion of polymorphic loci (P),
mean number of alleles per locus (A), mean number of alleles per polymorphic locus (Ap),
observed heterozygosity (Ho), expected heterozygosity (He), and population differentiation
(FST).
Site
State
Aldrich Lake
Ankeney Fen
Cambridge
Muck Lake
Mecosta Cnty
Sayles Road
Springville
Tuttle Marsh
Tyler Pond
Waushara
Michigan
Ohio
Wisconsin
Wisconsin
Michigan
New York
Ohio
Michigan
Maine
Wisconsin
Individuals Latitude Longitude
50
26
19
50
25
25
50
28
25
50
Table 1. Populations of Carex cryptolepis
sampled for this study.
Figure 1. Distribution of Carex cryptolepis Mack. (Cyperaceae) in North
America, showing populations already analyzed (green dots) and
populations to be sampled (blue dots). The southern maximal extent of
ice cover during the Wisconsin Glaciation is marked by the red line
(modified from Crins and Ball 1989).
FST
46.1275
39.7422
43.005
46.3922
43.7417
44.6371
41.0067
44.3958
44.3954
44.1518
86.2189
84.0053
89.0194
91.5533
85.3917
74.9382
83.4011
83.4214
69.8226
89.1642
Figure 3. CONTML tree illustrating tentative intraspecific and
interspecific relationships for Carex cryptolepis (green box) and C. lutea
(gold box). The relationships within C. cryptolepis are poorly resolved
by the current data set, but serve to guide further sampling efforts.
Works Cited
Bruederle, L.P., and D.E. Fairbrothers. 1986. Allozyme variation in populations of the Carex crinita
complex (Cyperaceae). Systematic Botany 11: 583-594.
------------------, and U. Jensen. 1991. Genetic differentiation of Carex flava and Carex viridula in West
Europe (Cyperaceae). Systematic Botany 16: 41-49.
Crins, W.J. and P.W. Ball. 1989. Taxonomy of the Carex flava complex (Cyperaceae) in North America
and Northern Eurasia. II. Taxonomic treatment. Canadian Journal of Botany 67: 1048-1065.
Felsenstein, J. 2005. PHYLIP (Phylogeny Inference Package) version 3.6. Distributed by the author.
Department of Genome Sciences, University of Washington, Seattle.
Ibrahim, K.M., R.A. Nichols, and G.M. Hewitt. 1996. Spatial patterns of genetic variation generated by
different forms of dispersal during range expansion. Heredity 77: 282-291.
Kuchel, S.D., and L.P. Bruederle. 2000. Allozyme data support a Eurasian origin for Carex viridula subsp.
viridula var. viridula (Cyperaceae). Madrono 47: 147-158.
Lewis, P. O., and D. Zaykin. 2001. Genetic Data Analysis: computer program for the analysis of allelic
data. Version 1.0 (d16c). Free program distributed by the authors over the internet from
http://lewis.eeb.uconn.edu/lewishome/software.html
Pielou, E.C. 1991. After the ice age: the return of life to glaciated North America. University of Chicago
Press, Chicago, IL.
Acknowledgments
Council Awards for Graduate Student Research provided
funding for this research.
Figure 2. Clump-forming growth form of Carex cryptolepis.